American Society of Clinical Oncologists (ASCO) and the College of American Pathologists (CAP) developed scoring criteria to standardize interpretation and ensure consistency of results between independent laboratories.
The 2018 ASCO-CAP scoring of HER2 by IHC is shown in the table.
| Result | Criteria |
| Negative (score 0) | No staining is observed or incomplete membrane staining, faint/barely perceptible, and in
≤10% of tumor cells |
| Negative (score 1+) | Incomplete membrane staining that is faint/barely perceptible and in >10% of tumor cells |
| Equivocal (score 2+) | Weak to moderate complete membrane staining observed in >10% of tumor cells *Required confirmatory Her-2 ISH testing |
| Positive (score 3+) | Circumferential membrane staining that is complete, intense, and in 10% of tumor cells |
